The present invention relates generally to apparatus and methods for measuring the dynamic response of physical structures, and more specifically to a system for measuring the spatial distribution of relative vibratory strain over the surface of a structure using an array of piezoelectric polymer film strain sensors coupled with a signal processor.
Knowledge of the spatial distribution of vibratory strain in a structure, such as an aircraft wing, is very useful in a design process. Areas of relatively high vibratory strain are likely locations for fatigue damage to occur. Fatigue damage can drastically reduce the service life of a structure, resulting in additional costs and downtime for repair or replacement of prematurely failed parts. Annual maintenance and repair of aircraft structures costs the U.S. Air Force hundreds of millions of dollars, making it one of the most significant post-production costs during the life cycle of aircraft. Determining the distribution of vibratory strain in these structures is an important step in diagnosing structural fatigue related problems and designing cost effective solutions. Experimental identification of these locations will permit design changes to reduce the strain or identify areas for application of damping treatments. Additionally, knowledge of strain mode shapes, the patterns of strain a structure exhibits while in resonant vibration, are extremely useful for improving mathematical design models of a structure.
No practical method exists in the prior art for measuring the absolute vibratory strain distribution over the surface of a structure. Conventional foil-type electrical resistance strain gages are capable of measuring dynamic strain at a single point on a structure. However, the cost and effort required to instrument the surface of a structure with an array of resistance strain gages in sufficient number to measure strain distribution are prohibitive.
It would be sufficient for many applications to measure the relative vibratory surface strain distribution on a structure instead of the absolute strain. Relative strain may be defined as proportional to absolute strain. Unfortunately, no practical method exists in the prior art to precisely measure relative strain distribution. Relative strain distribution can be mathematically estimated from the displacement mode shapes of a vibrating structure. However, this approach suffers from major drawbacks. First, it would be a complex and expensive task to measure the displacement mode shapes. Typically, this would require using a large number of accelerometers to obtain the dynamic displacement response of a test structure. Closely spaced measurement locations would be required to obtain smooth strain maps. Additionally, the mathematical estimation of strain shapes from displacement mode shapes can introduce significant errors. Because the mode shapes are proportional to displacement, they must be differentiated twice spatially to obtain a strain shape, adding more opportunities for computational noise and errors to enter into a final modal strain map estimate. The prior art typically uses measured displacement shapes only for updating a computerized finite element model and then obtains strain fields from the computerized model.
A more direct approach for obtaining strain shapes, or stain maps, for indicating dynamic strain distributions on structural surfaces is to use strain sensors to directly collect strain response data and use that data to identify the modal strain map. An alternative type of strain sensor from conventional electrical resistance strain gages has received much recent attention. Piezoelectric polymer films, usually made of polyvinylidene fluoride, or PVDF, have found recent use in such items as computer keyboards. See, for example, U.S. Pat. No. 4,975,616 to Park for a Piezoelectric Transducer Array. The Park patent expands on the more usual keyboard use of PVDF films as merely on-off switches.
The advantages of using PVDF film as a strain sensor are evident. They are lightweight, durable, can be shaped and stacked, have a high sensitivity to strain and can be isotropic in plane. Unfortunately, if used as a straight replacement for electrical resistance strain gages for obtaining strain maps, they suffer from many of the same deficiencies as electrical resistance strain gages.
Thus it is seen that there is a need for a successful adaptation of PVDF and other piezoelectric polymer films for use in measuring relative vibratory strain that utilizes the advantages of such films and avoids the disadvantages of other strain sensors that might be used for that purpose.
It is, therefore, a principal object of the present invention to provide a improved system for determining the dynamic response of physical structures using a piezoelectric polymer film.
It is a feature of the present invention that its incorporation of a strain sensor array onto a single sheet of piezoelectric polymer film provides a strain sensor system that is much easier, faster and less expensive to install than electrical resistance strain gages.
It is another feature of the present invention that, unlike electrical resistance strain gages, external excitation or electrical bridging is not required.
It is an advantage of the present invention that because an array of strain sensors can be produced on a single polymer sheet, a single strain sensor array replaces a large number of prior art strain sensors and the cost and effort of individually applying them.
It is another advantage of the present invention it requires no complex signal conditioning.
It is a further advantage of the present invention that its strain sensor arrays can be easily made in various sizes and smaller sizes can be easily made by simply cutting larger sizes with a scissors.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.